Satellite communication systems have been developed for transmitting information from a source to a destination. In these communication systems, an information signal is initially transmitted from the source to a geostationary satellite. The signal is received by the satellite, and then retransmitted to the destination. An antenna is utilized at the source to transmit the signal to the satellite, and at the destination to receive the retransmitted signal from the satellite. To ensure that the communication link is maintained, both the transmitting and receiving antennas must be pointed in the direction of the satellite. Geostationary satellites orbit the earth in such a manner that they maintain a constant orientation relative to any particular location on the earth's surface. Therefore, if the source and destination have fixed locations, the antenna pointing direction for each, once established, is essentially fixed. Consequently, for communication systems that have fixed transmitting and receiving locations, the satellite tracking system need not adjust the antenna pointing direction on a continuous basis in order to track the satellite.
Systems have also been developed for establishing a satellite communication link with a mobile body, such as a motor vehicle or a ship. When a satellite tracking system is installed on a ship, an antenna is mounted on the ship for transmitting signals to and receiving signals from a geostationary satellite. The antenna pointing apparatus for a shipborne tracking system is more complex than for stationary tracking systems because the pointing direction of the antenna, relative to the ship, continuously changes due to various factors associated with the orientation of the ship. As the ship travels from one location to another, it may change its heading direction relative to the earth which causes a corresponding change (yaw) in the desired pointing direction of the antenna relative to the ship. Additionally, as a result of disturbances in the water surface, the ship may pitch and roll, thereby tilting the ship relative to the satellite and requiring that a corresponding change be made in the pointing direction of the antenna relative to the ship in order to keep the antenna pointed in the direction of the satellite.
To maintain continuous communication with the satellite, it is desirable to ensure that, despite variations in heading direction, pitch and roll of the ship, the antenna remains continuously pointed in the direction of the satellite. A number of prior art systems have been developed for controlling the pointing direction of a ship-mounted antenna to compensate for the above-described factors. Many of these systems employ a turntable that is rotatable through 360 degrees about the azimuth axis, and an arm, carrying an antenna at its end, that is mounted on the turntable and is rotatable through 90 degrees about the elevation axis. By rotating the turntable through various degrees of azimuth and adjusting the arm through various degrees of elevation, the antenna pointing system can point the antenna at a satellite located anywhere in the hemisphere above the ship.
In these prior art antenna pointing systems, the turntable is mounted on a stabilized platform that keeps the turntable and the associated elevation arm at a level orientation, despite pitching and rolling of the ship. As a result, the turntable and elevation arm need only be adjusted to compensate for changes in the heading direction of the ship and in the elevation of the satellite in the sky in order to keep the antenna pointed in the direction of the satellite.
The platform can be stabilized either actively or passively. A passively stabilized platform utilizes gyroscopes that physically maintain the platform in a constant level orientation. Alternatively, actively stabilized platforms have been employed that use sensors to detect pitch and roll angles of the ship. These sensors are coupled to motors which drive gears that actively adjust the orientation of the platform relative to the ship to compensate for pitching and rolling thereof. In this manner, the platform is kept at a constant level orientation.
The above-described prior art systems suffer from several disadvantages. First, stabilized platforms, whether passive or active, are costly to implement. Passively stabilized platforms employ gyroscopes which are expensive. Actively stabilized platforms require the following four axes of control: (1) an axis to adjust the orientation of the platform relative to the ship to compensate for pitching of the ship; (2) an axis to adjust the orientation of the platform relative to the ship to compensate for rolling of the ship; (3) an azimuth control axis to adjust the orientation of the antenna to compensate for variations in the heading direction of the ship; and (4) an elevation control axis to adjust the orientation of the antenna to compensate for variations in the elevation of the satellite relative to the horizon. Each axis of control requires bearings, gears and a motor to drive the gears, thereby increasing the overall cost of the system.
A second problem associated with the turntable pointing systems is that a cable that couples the antenna to circuitry for transmitting and receiving signals to and from the satellite may wrap around the base of the system as the turntable is rotated. Since the cable has a finite length, the communication system may have to be periodically shut down to unwrap the cable. Some prior art designs overcome the cable wrap problem by coupling the cable to a slip ring that enables an electrical connection to be maintained with the antenna as the turntable is rotated. Although these designs overcome the cable wrap problem, the use of a slip ring introduces additional disadvantages because slip rings are costly and unreliable.
Other prior art systems have been developed which mount the turntable directly to the ship in an unstabilized manner. As the ship pitches, rolls and changes its heading direction, the turntable is rotated about the azimuth control axis and the elevation arm is adjusted along the elevation axis in an attempt to keep the antenna pointed in the direction of the satellite. Although these prior art systems use only two control axes, they suffer from a singularity of control because, at a particular location in the hemisphere above the antenna pointing apparatus where a satellite may be located, only one axis of control is effective to adjust the antenna pointing direction. For example, when the satellite is positioned directly over the ship, the elevation arm must be 90 degrees from horizontal to point the antenna in the direction of the satellite. As a result, rotation of the turntable about the azimuth control axis merely spins the antenna about this axis and does not alter the pointing direction of the antenna. The singularity of control is undesirable because it results in a lack of precision in the prior art antenna pointing system. The singularity of control complicates the antenna pointing arm adjustments that must be made about the two control axes as the orientation of the ship changes, thereby making it more difficult to keep the antenna pointed in the direction of the satellite.
Accordingly, it is an object of the present invention to provide an improved pointing apparatus.